HOT PLATE AND CONDUCTIVE PASTE

Abstract

 A hot plate having a conductive pattern layer that does not expand much and has superior adhesion. The hot plate (3) has a conductive pattern layer (10, 10a) arranged on a ceramic nitride substrate (9). The conductive pattern layer (10, 10a) includes bismuth or bismuth oxide, glass frit, and noble metal grains.

Description

TECHNICAL FIELD

[0001] The present invention relates to a hot plate, which uses a ceramic substrate, and a conductive paste.

BACKGROUND ART

[0002] During a semiconductor fabrication process, for example, when heating and drying a silicon wafer subsequent to the application of a photosensitive resin, a heating apparatus, which is referred to as a hot plate, is normally used.

[0003] Substrates made of ceramic, such as alumina, are often used to form a hot plate. A resistor, which functions as a conductive layer and which has a predetermined pattern, is formed on one side of the alumina substrate. A terminal connection pad is formed on part of the resistor. Such conductive layer is formed by applying, heating, and bonding an alumina substrate silver paste to the substrate. Subsequently, a terminal pin is soldered to the pad, and the terminal pin is connected to a power source by a wire. A silicon wafer, which is a heated subject, is placed on an upper surface of the hot plate. When the resistor is energized in this state, the silicon wafer is heated to 100°C or higher.

[0004] A conductive paste including 60wt% to 80wt% of silver, 1wt% to 10wt% of glass frit, the base of which is lead boron silicate, 1wt% to 10wt% of a binder, and 10wt% to 30wt% of a solvent is often used to form a conductive pattern layer (refer to Japanese Unexamined Patent Publication No. 4-300249). Glass frit, which is a secondary component, is especially required to obtain the optimal adhesion for the conductive pattern layer.

[0005] When applying the above conventional lead paste directly to a ceramic substrate, the following shortcomings occur. The heat produced when bonding the paste causes the oxides in the paste to react with the ceramic and if, for example, the ceramic is aluminum nitride, produces a large amount of gases, such as nitrogen gas. This is considered to occur mainly because of the large amount of lead oxide in the glass frit. In this case, the high pressure of the nitrogen gas produced during the bonding of the paste forces the nitrogen gas to pass through the grain boundary of the silver grains toward the exterior of the conductive pattern layer. As a result, the conductive pattern layer is apt to expand and the accuracy for forming the pattern decreases.

[0006] If the amount of glass frit added to the paste is decreased to an extremely low level, the undesirable effects of the lead oxide is reduced. This suppresses expansion for a certain degree. On the other hand, this increases the possibility of the conductive pattern having lower adhesion.

DISCLOSURE OF THE INVENTION

[0007] It is an object of the present invention to provide a hot plate having a conductive pattern layer that expands little, has superior adhesion, and has a large specific resistance and to provide a conductive paste optimal for the manufacturing of such hot plate.

[0008] To achieve the above object, a first perspective of the present invention provides a hot plate using a ceramic substrate provided with a conductive layer. The conductive layer includes bismuth or bismuth oxide, glass frit, and noble metal grains. Accordingly, the conductive layer includes bismuth or bismuth oxide, which are relatively easily oxidized and reduced in comparison to oxides that are included in the glass frit. In a conductive layer that includes such substance, expansion is suppressed even if the amount of added glass frit is large. Further, the increase in the amount of added glass frit (the added amount being 1wt% or greater relative to the noble metal grains) improves the adhesion of the conductive layer.

[0009] A second perspective of the present invention is a hot plate having a conductive layer in which the content of bismuth or bismuth oxide is 18wt% or less. If the content exceeds 18wt%, the bismuth oxide noble metal grains are separated. This would result in non-uniform resistance.

[0010] A third perspective of the present invention is a hot 'plate in which a ceramic substrate is a ceramic nitride substrate or a ceramic carbide substrate. The ceramic nitride substrate or the ceramic carbide substrate has superior thermal conductivity and tends to react with glass frit and produce gases. By using an aluminum nitride substrate, which has especially superior heat resistance and high thermal conductivity among the ceramic nitride substrates, a hot plate that can withstand usage under high temperatures is produced. Further, a silicon carbide substrate may be used as the ceramic carbide substrate.

[0011] A fourth perspective of the present invention is a hot plate having a conductive layer that contains glass frit, which includes zinc boron silicate. Glass frit including zinc boron silicate reacts with the nitride or the carbide in the ceramic substrate and produces nitrogen gas. Further, it is assumed that bismuth or bismuth oxide suppresses such reaction. Accordingly, a large amount of gas is not produced and expansion of the conductive layer does not occur even if a conductive layer made from a material using this component is employed.

[0012] A fifth perspective of the present invention is a hot plate provided with a conductive layer including noble metal grains selected from at least one of gold grains, silver grains, platinum grains, and palladium grains. The gold grains, silver grains, platinum grains, and palladium grains relatively resist oxidization even when exposed to high temperatures and have a sufficiently large resistance. Thus, the optimal conductive layer, which serves as a heating resistor, is easily produced.

[0013] A sixth perspective of the present invention is a hot plate having a conductive layer formed from bismuth or bismuth oxide, glass frit, noble metal grains, and an organic vehicle.

[0014] A seventh perspective of the present invention is a hot plate in which the content of the bismuth or bismuth oxide in the conductive layer is 18% or less.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Fig. 1 is a schematic cross-sectional view showing a hot plate unit according to one embodiment of the present invention.

[0016] Fig. 2 is a partial enlarged cross-sectional view showing the hot plate unit of Fig. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] A hot plate unit 1 according to one embodiment of the present invention will now be described with reference to Figs. 1 and 2.

[0018] The hot plate unit 1, which is shown in Fig. 1, includes a casing 2 and a hot plate 3.

[0019] The casing 2 is a cup-like metal member having an opened portion 4, the cross-section of which is round, located at its upper portion. The casing 2 does not have to be cup-like and may have an opened bottom. A hot plate 3 is attached to the opening 4 by means of a seal ring 14. A lead wire hole 7 for receiving current supplying lead wires 6 extends through the peripheral part of the bottom portion 2a of the casing 2.

[0020] The hot plate 3 of the present embodiment, which is formed from a ceramic substrate 9, is a low-temperature hot plate 3 used to dry a silicon wafer W1, to which a photosensitive resin is applied, at 50°C to 300°C.

[0021] It is preferred that a ceramic nitride substrate be selected as the ceramic substrate 9 since it has superior heat resistance and high thermal conductivity properties. More specifically, it is preferred that an aluminum nitride substrate, a silicon nitride substrate, a boron nitride substrate, or a titanium nitride substrate be selected. Among these substrates, it is most preferred that the aluminum nitride substrate be selected and next preferred that the silicon nitride substrate be selected. This is because these substrates have the highest thermal conductivity among the above ceramic nitrides.

[0022] The ceramic substrate 9 is disk-like, has a thickness of about 1mm to 100mm, and has a diameter that is slightly smaller than the outer dimension of the casing 2.

[0023] Referring to Figs. 1 and 2, a wiring resistor 10, which serves as a conductive pattern layer, is formed in a concentric or spiral manner on the lower surface of the plate-like substrate 9. Pads 10a are formed on an end of the wiring resistor 10. The wiring resistor 10 and the pads 10a are formed by printing, heating, and bonding a conductive paste (noble metal paste) P1 on the surface of the ceramic substrate 9. In the hot plate 3 of the present embodiment, the surface for heating the silicon wafer W1 is located on the opposite side of the conductive pattern layer formation layer, or on the upper surface. Such structure has an advantage in that a difference in temperature between locations does not occur in the hot plate 3 and in that the silicon wafer W1 is uniformly heated.

[0024] The wiring resistor 10 and the pads 10a of the present embodiment that are formed from the noble metal paste P1 includes noble metal grains as a main component and glass frit, or the like, as a secondary component. It is preferred that the noble metal grains used in the present embodiment have an average grain diameter of 6µm or less and be flake-like.

[0025] It is preferred that the flake-like noble metal grains be selected from one of gold grains (Au grains), silver grains (Ag grains), platinum grains (Pt grains), and palladium grains (Pd grains). These noble metals relatively resist oxidation even if they are exposed to high temperatures and have a sufficiently large resistance when energized and heated. These noble metals may be used alone or by combining two, three, or four of these metals as described below. The combinations include Ag-Au, Ag-Pt, Ag-Pd, Au-Pt, Au-Pd, Pt-Pd, Ag-Au-Pt, Ag-Au-Pd, Ag-Au-Pt, Au-Pt-Pd, Ag-Au-Pt-Pd.

[0026] Referring to Figs. 1 and 2, the basal end of a terminal pin 12, which is made of a conductive material, is soldered to each pad 10a. This electrically connects each terminal pin 12 to the wiring resistor 10. Sockets 6a, which are located on the distal end of the lead wires 6, are fit into the distal ends of the terminal pins 12. Accordingly, the temperature of the wiring resistor 10 increases and heats the entire hot plate 3 when current is supplied to the wiring resistor 10 via the lead wires 6 and the terminal pins 12.

[0027] An example of the procedures for manufacturing the hot plate 3 will now be briefly described.

[0028] A sintering-aid agent, such as yttria, and a binder are added as required to ceramic grains to prepare a mixture. The mixture is uniformly kneaded into three rolls. The kneaded material is used to press mold plate-like molding products having a thickness of 1 to 100mm.

[0029] Holes are punched or drilled in the molded product to form pin insertion holes, which are not shown in the drawings. After the hole forming process, the molded product is dried. Then, the molded product undergoes provisional baking and main baking so that it is completely sintered. This forms the ceramic sinter substrate 9. It is preferred that the baking process be performed in a hot-press apparatus and that the baking process be performed at a temperature of about 1500°C to 2000°C. Afterward, the ceramic substrate 9 is cut into a disk-like shape having a predetermined diameter (in the present embodiment, 230mmΦ) and undergoes surface grinding with a hub grinder.

[0030] After the above process, the noble metal paste P1, which has been prepared beforehand, is uniformly applied to the lower surface of the ceramic substrate 9, preferably through screen-printing.

[0031] In addition to noble metal grains, the noble metal paste P1 used here includes ruthenium oxide, glass frit, a resin binder, and a solvent. The noble metal paste P1 may also include bismuth or bismuth oxide.

[0032] The reason for adding bismuth (Bi) or bismuth oxide (Bi₂O₃) to the noble metal paste P1 is as follows. Test results have shown that by adding these substances, reaction between the glass frit and the aluminum nitride or the silicon carbide is suppressed and the adhesion of the wiring resistor 10 and the pads 10a is increased. These substances are relatively easily oxidized and reduced in comparison to other oxides. It is presently presumed that such properties contribute in one way or another to suppress expansion and enhance adhesion.

[0033] When selecting, for example, aluminum nitride, as the substrate material, bismuth oxide reacts with the aluminum nitride when the paste is bonded and produces alumina and nitrogen gas. Thus, the bismuth oxide functions as an oxidization agent of the aluminum nitride. Further, when exposed to air, bismuth is easily oxidized into bismuth oxide. Thus, bismuth may considered as an indirect oxidization agent of the aluminum nitride.

[0034] Additionally, when selecting, for example, silicon nitride as the substrate material, bismuth oxide reacts with silicon nitride when the paste is bonded and produces silica and nitrogen gas. Thus, the bismuth oxide functions as an oxidization agent of the silicon nitride. In the same manner, bismuth may be considered as an oxidization agent of the silicon nitride.

[0035] It is preferred that about 0.1wt% to 10wt% of bismuth or bismuth oxide be included in the noble metal paste P1, more preferred that about 1wt% to 5wt% be included, and especially preferred that about 2wt% to 3wt% be included. If the content of bismuth or bismuth oxide is too small, the effect obtaining by adding bismuth or bismuth is insufficient. Thus, the expansion may not be prevented and the adhesion may not be significantly improved. On the other hand, if the content of bismuth and bismuth oxide is too large, reaction that generates nitrogen gas increases. This may increase expansion.

[0036] It is preferred that the amount of glass frit be a fraction of the amount of noble metal grains. This is because such amount of the glass frit component in the noble metal paste does not generate much nitrogen gas and the adhesion of the wiring resistor 10 and the pads 10a do not decrease. Further, as the amount of a conductive component in the noble metal paste P1 increases, the specific resistance of the wiring resistor 10 may be decreased. Specifically, in the present embodiment, 60wt% to 80wt% of noble metal grains and 1wt% to 10wt% of glass frit is included in the noble metal paste P1.

[0037] It is preferred that glass frit including zinc boron silicate (SiO₂: B₂O₃: ZnO₂) be used, and especially preferred that the glass frit includes zinc boron silicate as a base (i.e., main component). More specifically, it is preferred that a small amount of oxide be added to the zinc boron silicate, which serves as a base. Specific examples of oxides include aluminum oxide (Al₂O₃), yttrium oxide (Y₂O₃), lead oxide (PbO), cadmium oxide (CdO), chromium oxide (Cr₂O₃), and copper oxide (CuO). One of these oxides or a combination of two or more of these oxides may be added to the zinc boron silicate. During the bonding of the paste, these oxides function as an oxidization agent of the substrate material and are thus reduced.

[0038] It is preferred that the weight ratio of each of the above listed oxides be 1/20 times to 1/5 times the weight ratio of zinc boron silicate. If the weight ratio is too small, the percentage of the above oxides in the glass frit increases. As a result, the expansion caused by nitrogen gas may not be sufficiently prevented. On the other hand, if the weight ratio is too large, the percentage of the above oxides in the glass frit decreases. As a result, the adhesion of the wiring resistor 10 may not be sufficiently increased.

[0039] The noble metal paste P1 also includes 3wt% to 15wt% of a resin binder, which serves as an organic vehicle, and 10wt% to 30wt% of a solvent. Examples of the resin binder are, for example, the cellulose group such as ethyl cellulose. The solvent is a component added to improve the printing and dispersion characteristics. Specific examples of the solvent are the acetate group, the cellosolve group such as butyl cellosolve, or the Carbitol group such as butyl Carbitol. One or a combination of two or more of these solvents may be used.

[0040] When the noble metal paste P1 applied to the ceramic substrate 9 is heated for a predetermined time at a temperature of about 750°C, the solvent in the noble metal paste P1 volatilizes and bonds the wiring resistor 10 and the pads 10a to the ceramic substrate 9. Fused glass frit has a tendency to move toward the surface of the ceramic substrate 9. Contrarily, the noble metal grains have a tendency to move away from the surface of the ceramic substrate 9.

[0041] Subsequently, the pads 10a are connected to the terminal pins 12 by a solder S1 to complete the hot plate 3. Then, the hot plate 3 is attached to the opening 4 of the casing 2 to complete the desired hot plate unit 1 of Fig. 1. Thus, the resistor of the hot plate unit 1 does not expand and has high tensile strength. Further, the difference in the resistance of the resistor is small. This uniformly heats the heating surface of the hot plate.

(Examples and Comparative Examples)

[Preparation of Samples (Metal Species of the Noble Metal Grains Being the Same)]

[0042] In examples 1 to 5 and comparative examples 1 to 3, 4 parts by weight of Y₂O₃ (average grain diameter 0.4µm) and 8 parts by weight of an acrylic resin binder (manufactured by Mitsui Chemicals, product name: SA-545, acid number 1.0) were added to 100 parts by weight of aluminum nitride powder (average grain diameter 1.1µm) and mixed. The mixture produced in this manner was uniformly kneaded. The kneaded product was put into a press mold and pressed to form a plate-like molded product.

[0043] Then, after forming holes and performing a drying process, the molded product was degreased in a nitrogen atmosphere for four hours at a temperature of 350°C for four hours to thermally decompose the binder. Further, the degreased molded body was baked in a hot-press for three hours at a temperature of 1600°C to produce an aluminum nitride substrate. The pressure of the hot press was 150kg/cm².

[0044] Then, after cutting the substrate and performing surface grinding, a paste applying process was performed. In the process, the noble metal paste P1, the composition of which is described below, was used and applied to a thickness of about 25µm. Eight types of samples were prepared in accordance with the above procedure (refer to table 1).

[0045] Only one type of noble metal grains, that is, silver grains, which were flake-like and had an average grain diameter of 5µm, was used. The added amount of the silver grains in the silver paste, which served as the noble metal paste P1, was 65wt% in samples 2 and 7 and 70wt% in the other samples.

[0046] Four types of glass frit including zinc boron silicate as a base (i.e., a zinc-containing material was used as the glass frit) and one type of glass frit including lead boron silicate (i.e., a lead containing material) was prepared. The specific compositions of zinc glass frits α, β, γ, δ are each shown in the lower rows of table 1. The amount of each glass frit added to the noble metal paste is as shown in table 1.

[0047] The added amount of bismuth in samples 1, 3, 4, and 5 (i.e., examples 1, 3, 4, and 5) is set at 3wt%, the added amount of bismuth in sample 2 (i.e., example 2) is set at 2wt%, and the added amount of bismuth in the other samples (comparative examples 1, 2, 3) is set at 0wt%.

[0048] Ethyl cellulose was selected as the binder, and butyl Carbitol was selected as the solvent. The added amount of ethyl cellulose was 5wt% and the added amount of noble metal paste P1 was 15wt%.

[0049] Although bismuth was added, ruthenium oxide was not added in samples 6, 7, and 8. Thus, samples 6, 7, 8 do not satisfy the optimal conditions of the present embodiment. Further, in sample 8, the amount of glass frit is small in comparison to the amount of silver grains. Thus, sample 8 does not satisfy the optimal conditions of the present embodiment. Accordingly, samples 1 to 5 correspond to examples 1 to 5, and samples 6 to 8 correspond to comparative examples 1 to 3.

[Comparison Test and Results]

[0050] In each of the eight samples, the paste was printed to and bonded on the ceramic substrate 9, and two square millimeter test patterns were formed at multiple locations. A tensile strength test was performed on test patterns that did not expand, and the average value of the measured values (kgf/2mm□) was calculated. The expansion of the test patterns was confirmed through observation with the naked eye and with an optical microscope. Further, voltage was applied to increase the temperature of the samples to 180°C. Then, the difference (°C) between the maximum temperature and the minimum temperature in the heating surface was confirmed with a thermo-viewer (IR-62012-0012, manufactured by Nihon Datum). The results of the tests are shown in table 1.

Table 1

Sample No.	Grains (wt%)	Added Amount of Bi or its oxide (wt%)	Type and Added Amount of Glass Frit (wt%)	Expansion	Tensile Strength (kgf/2mm□)	Temperature Difference (°C)
1 (Example 1)	Ag 70	3	α (Zn-Containing), 3	None	12.2	0.5
2 (Example 2)	Ag 65	2	α (Zn-Containing), 5	None	11.8	0.4
3 (Example 3)	Ag 70	3	β (Zn-Containing), 3	None	9.4	0.5
4 (Example 4)	Ag 70	3	γ (Zn-Containing), 3	None	9.8	0.5
5 (Example 5)	Ag 70	3	δ (Zn-Containing), 3	None	10.1	0.4
6 (Comparative Example 1)	Ag 70	0	α (Zn-Containing), 3	Confirmed	5.2	0.4
7 (Comparative Example 2)	Ag 65	0	α (Zn-Containing), 5	Confirmed	4.8	0.4
8 (Comparative Example 3)	Ag 70	0	Pb-Containing, 3	Confirmed	-	0.5
9 (Example 6)	Ag 56.6 Pd 10.3	2.1	Zn-Pb Containing	None	10.0	0.5
10 (Example 7)	Ag 56.6 Pd 10.3	15.1	Zn-Pb Containing	None	9.5	0.9
11 (Example 8)	Ag 56.6 Pd 10.3	25.0	Zn-Pb Containing	Confirmed	5.8	5.0
(Note) α: includes 80wt% of zinc boron silicate and 20wt% of Al2O3 β: includes 80wt% of zinc boron silicate, 10wt% of Al2O3, and 10wt% of Cr2O3 γ: includes 90wt% of zinc boron silicate, 5wt% of PbO, and 5wt% of CdO δ: includes 85wt% of zinc boron silicate and 15wt% of Cr2O3

[0051] As apparent from table 1, in examples 1 to 5, absolutely no expansion was confirmed and the pattern formation accuracy was superior. Further, the tensile strength values were extremely high, each value exceeding 9kgf/2mm□.

[0052] In comparison example 3, expansion was confirmed and the pattern formation accuracy was unsatisfactory. In comparison examples 1 and 2, although expansion was not confirmed, the tensile strength was only half of that of the values of examples 1 to 5. Accordingly, it was proved that the adding of a small amount of bismuth was extremely effective for improving the tensile strength.

[Preparation of Sample 9 (Metal Species of the Noble Metal Grains Being Different)]

[0053] In example 6, 45 parts by weight of Y₂O₃ (average grain diameter 0.4µm), 15 parts by weight of Al₂O₃ (average grain diameter 0.5µm), 20 parts by weight of SiO₂ (average grain diameter 0.5µm), and 8 parts by weight of an acrylic resin binder (manufactured by Mitsui Chemicals, product name: SA-545, acid number 1.0) were mixed with 45 parts by weight of silicon nitride powder (average grain diameter 1.1µm).

[0054] The mixture obtained in this manner was uniformly kneaded. The kneaded product was put into a press mold and pressed to form a plate-like molded product.

[0055] Then, after forming holes and performing a drying process, the molded product was degreased for four hours at a temperature of 350°C for four hours in a nitrogen atmosphere to thermally decompose the binder. Further, the degreased molded body was baked in a hot-press for three hours at a temperature of 1600°C to produce a silicon nitride substrate, or the ceramic substrate 9. The pressure of the hot press was 150kg/cm².

[0056] Then, after cutting the substrate and performing surface grinding, a paste applying process was performed. In the process, the noble metal paste P1, the composition of which is described below, was used and applied to a thickness of about 25µm to form sample 9. Bismuth oxide was used instead of bismuth.

Noble metal grains: 56.6 parts by weight of silver grains (Ag-520 manufactured by Shoei Chemical Inc) and 10.3 parts by weight of palladium grains (Pd-213 manufactured by Shoei Chemical Inc.)

Glass Frit: 1.0 parts by weight of SiO₂, 2.5 parts by weight of B₂O₃, 5.6 parts by weight of ZnO, and 0.6 parts by weight of PbO

Bi₂O₃: 2.1 parts by weight

Resin binder: 3.4 parts by weight

Solvent: 17.9 parts by weight of butyl Carbitol

[0057] The applied noble metal paste P1 was heated at a temperature of about 750°C for a predetermined time to bond the wiring resistor 10 and the pads 10a and complete the hot plate 3 of example 6.

[0058] In examples 7 and 8, 0.5 parts by weight of C (carbon) and 8 parts by weight of an acrylic resin binder (manufactured by Mitsui Chemicals, product name: SA-545, acid number 1.0) were mixed with 45 parts by weight of silicon carbide powder (average grain diameter 1.1µm).

[0059] The mixture obtained in this manner was uniformly kneaded. The kneaded product was put into a press mold and pressed to form a plate-like molded product.

[0060] Then, after forming holes and performing a drying process, the molded product was degreased.for four hours at a temperature of 350°C for four hours in a nitrogen atmosphere to thermally decompose the binder. Further, the degreased molded body was baked in a hot-press for three hours at a temperature of 900°C to produce a silicon nitride substrate, or the ceramic substrate 9. The pressure of the hot press was 150kg/cm².

[0061] A paste applying process was performed using the noble metal paste P1 (i.e., pastes A and B), the composition of which is described below to form samples 10 and 11 (examples 7 and 8).

<Paste A>

[0062] 

Noble metal grains: 56.6 parts by weight of silver grains (Ag-520 manufactured by Shoei Chemical Inc) and 10.3 parts by weight of palladium grains (Pd-213 manufactured by Shoei Chemical Inc.)

Glass Frit: 1.0 parts by weight of SiO₂, 2.5 parts by weight of B₂O₃, 5.6 parts by weight of ZnO, and 0.6 parts by weight of PbO

Bi₂O₃: 15.1 parts by weight

Resin binder: 3.4 parts by weight

Solvent: 17.9 parts by weight of butyl Carbitol

<Paste B>

[0063] 

Noble metal grains: 56.6 parts by weight of silver grains (Ag-520 manufactured by Shoei Chemical Inc) and 10.3 parts by weight of palladium grains (Pd-213 manufactured by Shoei Chemical Inc.)

Glass Frit: 1.0 parts by weight of SiO₂, 2.5 parts by weight of B₂O₃, 5.6 parts by weight of ZnO, and 0.6 parts by weight of PbO

Bi₂O₃: 25.0 parts by weight

Resin binder: 3.4 parts by weight

Solvent: 17.9 parts by weight of butyl Carbitol

[Comparison Test and Results]

[0064] The same comparison test as that conducted on examples 1 to 5 and comparison examples 1 to 3 was performed on samples 9, 10, and 11 corresponding to examples 6, 6, and 8. Expansion of the wiring resistor 10 and the pads 10a was not confirmed in examples 6 and 7. Further, in example 8, in addition to the confirmation of the expansion, the temperature difference in the heating surface was 5°C and large.

[0065] Accordingly, the examples of the present embodiment have the advantages described below.

(1) In the hot plates 3 of examples 1 to 5, the wiring resistor 10 and the pads 10a are formed from bismuth, glass frit, and silver grains. Further, in the hot plates of examples 6 and 7, the wiring resistor 10 and the pads 10a re formed from bismuth oxide, glass frit, silver grains, and palladium grains.
Accordingly, expansion is suppressed and adhesion is improved without reducing the added amount of glass frit. Thus, the hot plate 3 has superior pattern formation accuracy and high reliability.
In examples 1 to 5, the bismuth in the noble metal paste P1 may be replaced by the same amount of bismuth oxide. In examples 6 and 7, the bismuth oxide in the noble metal paste P1 may be replaced by the same amount of bismuth.

(2) In examples 1 to 5, an aluminum nitride substrate, which has especially superior heat resistance and high thermal conductivity, is used as the ceramic substrate 9. Thus, the hot plate 3 is practical since it may be used under high temperatures.

[0066] The embodiment of the present invention may be modified as described below.

[0067] Spherical noble metal grains may be used in lieu of the flake-like noble metal grains. Further, instead of using only one type of the noble metal grains, two or more types of noble metal grains (e.g., flake-like grains and spherical grains) may be mixed and used.

[0068] The ceramic substrate 9, which is formed from aluminum nitride or silicon nitride, is not limited to products manufactured through press molding and may be manufactured, for example, by performing sheet molding with a doctor blade apparatus. When performing sheet molding, the wiring resistor 10 may, for example, be arranged between superimposed sheets. Thus, the high temperature hot plate 3 is manufactured in a relatively simple manner.

[0069] The conductive pattern layer is not limited to the wiring resistor 10 and the pads 10a used in the above embodiment and may be other structures such as a conductive pattern layer that is not a heating resistor.

[0070] The noble metal paste P1 need not be screen printed on the ceramic substrate 9. For example, the noble metal paste P1 may be stamped on the ceramic substrate 9.

[0071] The above oxides do not have to be included in the noble metal paste P1 separately from glass frit and may be included in the noble metal paste P1 in a state in which the oxide is added to the glass frit as a secondary component of the glass frit. Oxides included in the glass frit as a secondary component is more preferred since such oxide is uniformly dispersed in the noble metal paste P1.

Claims

1. A hot plate having a ceramic substrate provided with a conductive layer, the hot plate characterized in that the conductive layer includes bismuth or bismuth oxide, glass frit, and noble metal grains.

2. The hot plate according to claim 1, characterized in that the content of bismuth or bismuth oxide is 18wt% or less.

3. The hot plate according to claim 1 or 2, characterized in that the ceramic substrate is a ceramic nitride substrate or a ceramic carbide substrate.

4. The hot plate according to any one of claims 1 to 3, wherein the glass frit includes zinc boron silicate.

5. The hot plate according to any one of claims 1 to 3, wherein the noble metal grains is at least one selected from a group consisting of gold grains, silver grains, platinum grains, and palladium grains.

6. A conductive paste characterized in that the conductive paste includes bismuth or bismuth oxide, glass frit, noble metal grains, and an organic vehicle.

7. The conductive paste according to claim 6, wherein the content of bismuth or bismuth oxide is 18wt% or less.

Drawing